The invention relates to a method of measuring a measurable characteristic, of an object to be measured to which an auxiliary variable is applied and which is coupled with a measured value pickup or sensor element which emits a directly measurable, analog measurement signal corresponding to the measurable variable, with a conclusion being made as to the measurable variable from the auxiliary variable and from the measurement signal. The invention also relates to use of the method for measuring electrical and non-electrical variables and to an apparatus for implementing the method.
A method of the mentioned type for measuring nonelectrical variables is disclosed in DE-OS No. 3,333,129 in which the non-electrical variable, such as force, pressure and temperature, is measured by way of the deformation of a conductor, with the variable to be measured acting on an interdigital transducer which receives a coded acoustic surface wave that is decoded by means of correlation reception and evaluated with respect to its correlation maximum. The coded surface wave signal may be a binary phase keyed signal or a chirp signal. For this purpose, two interdigital transducers are applied to a piezoelectric substrate, one transducer being configured as a signal transmitter and the other transducer as a filter for the reception of the transmitted signal. The receiving filter constitutes a matched filter for the transmitted signal and is configured to receive, by respective weighting of the transducer, either the binary phase keyed signal or the linearly frequency modulated chirp signal. If no non-electrical measurable variables act on the interdigital transducer structure of the matched filter, the autocorrelation function with major and minor peak ratio is measured at the output of the filter. If, however, corresponding measurable variables do act on the interdigital transducer, the geometry of the transducer and consequently the transfer function of the transducer are changed and the signal generated by the transmitting transducer is received in a mismatched way. This is expressed in a reduction of the autocorrelation maximum and can thus be evaluated electrically.
A method of determining momentary flow rates is known from Published Patent Application No. (without examination) DE 3,106,530 A1. Here, pseudo-randomly marked flow medium particles are utilized for transit time measurements. For this purpose, the position of the maximum of a cross-correlation function is employed.
The publication by W. D. T. Davies, entitled, "System Identification for Self-Adaptive Control," John Wiley and Sons, Ltd. London, 1970, discloses pseudo-random signals of maximum length. MBS signals are known from A. Van den Bos, "Construction of Binary Multifrequency Test Signals," IFAC Symp., Prague, 1967.
DE-OS No. 3,015,519 discloses a method of determining the capacitance of an object being measured to which an interfering direct voltage is additionally applied and in which the time constant fixed by a charging or discharging process is determined, and from it, the desired capacitance is determined. The determination of the time constant is performed by three current measurements with the second measurement beginning in an integrating manner when two measuring resistors are connected to the object to be measured. A circuit arrangement composed of a dipole including the two measuring resistors serves to implement this method with a switch associated with the measuring resistors being provided in the dipole to permit the selective connection of the respective measuring resistor. The two measuring resistors may here be connected either in parallel and activated by a switch, or they may be connected in series and one of the measuring resistors may be selectively bridged by a switch.
Also known are methods and apparatus for measuring resistances, capacitances and inductances, with such methods and apparatus being divided, in principle, into four different groups:
1. The value of the component to be analyzed is determined by means of a bridge circuit. In this method, disclosed in the publication "Profos: Handbuch der industriellen Messtechnik" [Profos: Handbook of Industrial Measuring Technology], published by Vulkan-Verlag Essen; 3rd Edition, Essen, 1984, the supply voltage for the bridge circuit is composed either of a sinusoidal voltage or of a direct voltage. The measured value may be determined by matching the bridge or from the bridge voltage. The problem in this method is that the supply voltage must either have a very good sinusoidal shape with constant amplitude and frequency, i.e. a low harmonic distortion factor, or it must be designed as an accurate direct voltage free of drifts.
2. The component to be measured is installed in an oscillator as a frequency determining element. In this method, which is disclosed, for example, in DE-OS No. 2,816,655, there exists the drawback that the frequency of the oscillator must be constant during the measuring period while it may vary from measurement to measurement.
3. The component to be measured is connected with a known component to form a delay member so as to provide a conclusion as to the value of the component to be measured from the charging process and/or the discharging process, as disclosed, for example, in the above-mentioned DE-OS No. 3,015,519. Here, either the time is measured until the current in the circuit or the voltage across the circuit has reached a certain value, or the current in the circuit or the voltage across the circuit is measured after a fixed time. The drawback is that an offset in the measured variable cannot be eliminated or only imprecisely determined, a noise component contained in this variable, such as a drift, which occurs linearly over time, is not eliminated and sometimes very long measuring times must be accepted.
4. The impedance of the component to be measured is determined, in which case similar problems arise as in the measuring method mentioned under Point 1.